New, Brian (2006) Controls of copper and gold distribution ...

This file is part of the following reference:

New, Brian (2006) Controls of copper and gold distribution in the Kucing Liar deposit, Ertsberg mining district, West Papua, Indonesia.

PhD thesis, James Cook University.

Access to this file is available from:

http://eprints.jcu.edu.au/2083

http://eprints.jcu.edu.au/2083

82

4 Structural setting

The following section documents the large-scale context of the Kucing Liar mineralisation, as

reflected by the distribution of the major hydrothermal minerals. Routine logging of all drill core

included identification of lithology and estimation of individual mineral abundances for all core

samples (Appendix IV). During routine exploration drilling, sample intervals were assigned by

mine geologists at regular lengths of 3m but made shorter where significant changes in geology

were present (N. Wiwoho, pers comm.). Technicians then split the core using a screw press to

produce a sample ready for assay. A short length of drill core (10-20cm) was removed from each

assayed interval prior to splitting, and retained, producing an archive of “skeleton” core. The drill

holes were logged in two phases from Sept.-Nov. 1997 and June-Sept. 1999. During the first

phase, continuous (“full core”) that had been split for sampling was utilised, while during the

second period only skeleton core was examined due to the much shorter time required to examine

and log a single hole. The second period of logging revisited many of the drill holes logged on the

first occasion, enabling a comparison between logs of continuous core versus skeleton core. No

major differences were found comparing the data from the two different sample collections.

The structural setting has been interpreted independently for this research program from the

skeleton core logged by this author to develop detailed cross sections through the mineralised

zone. Polygonal outlines developed from data within individual drilling stations for major

stratigraphic contacts each of the major mineral assemblages honour the position of contacts on

drillhole traces and were converted into wireframes. Three-dimensional surfaces representing the

major stratigraphic contacts have been derived from correlation of stratigraphy between radial

drill fans (Chapter 1), and are used to illustrate the structural setting of mineralisation. Visual

estimates of mineral abundances in drill core samples (see Appendix V) were also analysed in

three dimensions using Vulcan and Surpac mine-environment software. Cross sections of these

surfaces and isosurfaces are the primary method used to interpret structural controls on fluid flow.

83 Structural setting _____________________________________________________________________________

4.1 STRATIGRAPHIC AND FLUID FLOW MODELLING

The main units of interest in the Kucing Liar deposit are the Ekmai and Waripi Limestones, and

less importantly the Faumai Limestone, the Sirga Sandstone and the Kais Limestone. Each of

these units is unique with respect to the overall sequence. Their contacts are important in

identifying stratigraphical position as well as the location of fault zones. The positions of the

distinctive marker horizons have been correlated between drill stations to form three-dimensional

surfaces and are combined with the 3D distribution of hydrothermal alteration to provide as full a

picture as possible of the structural history of the Kucing Liar system.

4.1.1 Lithological distribution

Stratigraphic sequence recognition

While the original composition and texture of wall rocks could be identified for type samples in

KLS1-1 and KLS3-1, much of the sequence is affected by hydrothermal alteration. As the wall

rocks in the mineralised zone are, by definition, extensively replaced, identification of texture

retention during alteration is an important step in correctly assigning stratigraphic position of host

rocks. Textural retention and lithological control of mineralogy is a feature of hydrothermal

mineral development at Kucing Liar, which, in combination with unique sequences and marker

horizons, allows stratigraphic characterisation of the altered sequences in the main mineralised

zone. As the sequence of lithology has been established, deviations from the expected lithology

identify the components added during modification. Each type of alteration displays a continuum

of modification that can be traced from original texture and composition in KLS1-1 and KLS3-1

through to total replacement of the original rock within the mineralised zone.

Textural retention during alteration

Fundamentally, there are only three sedimentary rock types, i.e. limestone, sandstone and shale,

which host Kucing Liar alteration and mineralisation, each of which can be identified either by a

distinctive texture or a particular alteration mineralogy (Plate 4-1).

84 Kucing Liar, Ertsberg mining district _____________________________________________________________________________

The base of the mineralised zone is marked by the Ekmai Sandstone which is a relatively

homogeneous unit with monotonous white K-feldspar ± muscovite ± covellite ± pyrite. In some

drilling, sharp contacts between K-feldspar and quartz-dominant alteration were observed. The

deepest penetration of the Ekmai Sandstone (in KL42-3) intersected a section of clinopyroxene-

garnet-K-feldspar-biotite hornfels that is very similar in appearance to altered sections of the

Ekmai Limestone. The lower contact with the Ekmai Sandstone marks a change from underlying

homogeneous sandstone to very fine-grained shale and is distinctive in altered sequences where

the overlying zone is typified by a 5-10m zone of abundant green or brown-red garnet and

magnetite in the lower Ekmai Limestone. The main body of the Ekmai Limestone is generally

homogeneous and typified by very fine-grained hornfels that may be green (pyroxene-feldspar),

white (K-feldspar) or brown (K-feldspar ± biotite). Where present, the Ekmai shale is altered to a

distinctive brown K-feldspar ± biotite rock and contains a quartz stockwork that is distinct from

sheeted vein arrays hosted in the underlying Ekmai Limestone and which are absent from

overlying Waripi Limestone (zone 8 in Plate 4-1). This unit is not present throughout the

mineralised zone, but it is definitive.

The Waripi Limestone overlies the Ekmai Limestone and typically contains a number of different

zones where intersected in the deposit. The lowermost zone commonly hosts thick concentrations

of magnetite or garnet that form sharp contacts with clinopyroxene-plagioclase or K-feldspar-

biotite altered shale. This zone is typified by intense brecciation with an abrupt lower contact to

the Ekmai Limestone and a transitional contact to overlying skarn alteration (zones 5 and 6 in

Plate 4-1). The lower contact with the Ekmai Limestone represents a change from very fine-

grained black shale to fine-grained grey peloid limestone and as such is easily recognised.

Diopside skarn occurs above the magnetite breccia /garnet zone and has been overprinted by

orange humite-forsterite, dark green phlogopite, green tremolite-actinolite or thin magnetite zones

(zone 4 in Plate 4-1). The upper contact of skarn and accompanying alteration is commonly sharp,

in some cases (zones 3 and 4 in Plate 4-1) defined by a thin concentration of garnet accompanied

by sphalerite and galena mineralisation. A zone of calcite alteration above this contact is


gradational to grey dolostone (zone 3 in Plate 4-1). This is succeeded by the upper Waripi

sandstone member, which is typically intensely altered to vuggy quartz and contains metre-scale

lenses of pyrite. This alteration does not typically extend outside the quartz layer, which is

overlain by thin K-feldspar-biotite hornfels developed in thin shale.

Marker horizons

The precise position of the upper Waripi Limestone contact with the Faumai Limestone is

difficult to recognise as the thin (~5m) laminated sandstone layer that marks the top of the Waripi

Limestone is commonly not observed due to the scale of sampling (see Appendix V). However,

the approximate location of the contact is indicated by the location of the much thicker (~50m)

upper Waripi sandstone member and its accompanying, easily identifiable shale layer (zones 2

and 12 in Plate 4-1). The position of the Idenberg Fault Zone can be identified in many drill holes

due to variation from the normal stratigraphic sequence. The Ekmai Sandstone is known from

regional studies to be 600m thick (Chapter 1). Drill holes oriented from near vertical or toward

the northeast either intersect thick monotonous sandstone or porphyry. However, drill holes

oriented toward the southwest do not follow the expected sequence, indicating the presence of a

fault. The Idenberg Fault Zone is manifested either as an abrupt change in lithology, which may

be altered sandstone to altered limestone, commonly separated by 5 to 10m of magnetite ±

phlogopite ± tremolite ± chalcopyrite ± pyrite, or as a broader zone characterised by fragmental

rocks altered to magnetite ± quartz ± pyrite (zone 9 in Plate 4-1). In many instances the alteration

mineralogy does not allow specific identification of the texture or lithology of the precursor.

Garnet, magnetite, quartz and pyrite are all associated with extensive fragmentation of the host

rocks (Chapter 3) and it is interpreted that intense development of these minerals indicates the

presence of a fractured zone that represents the position of a fault (Plate 4-1).

Out-of sequence limestone has been encountered beneath the Ekmai Limestone on the southwest

margin of the main mineralised zone. A thick magnetite zone commonly occupies the contact

between normal stratigraphy and out-of-sequence limestone (zone 9 in Plate 4-1). The limestone


is variably altered with increasing intensity with depth from calcite to calcite ± magnetite with

minor humite development and finally to clinopyroxene and humite skarn overprinted by

retrograde tremolite-actinolite and serpentine (zones 10 and 11 in Plate 4-1). The distinctive upper

Waripi sandstone member is occasionally recognised in deeper drilling intersections within this

zone of altered limestone from the same distinctive quartz and potassic hornfels alteration

identified above the mineralised zone (e.g. zone 12 in Plate 4-1). Additionally, sandstone of

similar thickness to the upper Waripi sandstone member but containing a shale layer in the middle

rather than at the top is identified in a small number of holes and is interpreted to be the Sirga

Sandstone. Where recognised, the Sirga Sandstone is quartz-garnet altered with minor lenses of

pyrite. Recognition of these distinctive layers in deeper drilling confirms the presence of a fault

zone and allows the magnitude of displacement to be measured.

Stratigraphical interpretations indicate that the Kucing Liar deposit can be divided into two parts

with similar sequences separated and offset by the Idenberg Fault Zone. The position of this fault

is indicated by departure from the ideal stratigraphic sequence (Figure 4-1). The rocks above the

fault consist of well-constrained stratigraphic sequence of Waripi Limestone, Ekmai Limestone

and Ekmai Sandstone and will be referred to as the main mineralised zone. The footwall

stratigraphy is less well defined due to limited drilling. The general impression of the structure at

Kucing Liar can be gained by comparing the sequences intersected at different drill stations along

the strike of the system (Figure 4-1). The stratigraphy of Kucing Liar is generally consistent along

strike though there is a significant change in the structure at station KL44, which marks the

western extent of mineralisation (Figure 4-1). Below the fault zone the Waripi Limestone and

Faumai Limestone are recognised from the relative position of a second intersection of the upper

Waripi sandstone member (Figure 4-1). The Sirga Sandstone is also recognised in KL44-2

adjacent to the main mineralised zone but separated from it by a porphyry intrusion that occurs in

the Idenberg Fault Zone (Figure 4-1).


Plate 4-1 An example of lithological sequence commonly found in faulted regions

Drill core samples from KL40-07 (Figure 4-1) representing assay intervals that are generally 3m long. The

total depth of the hole is 911m (lst = limestone, shl = shale, sst = sandstone, unk = unknown). Zones: 1–

carbonate altered limestone, 2–quartz altered sandstone including feldspar + biotite altered shale, 3–

carbonate altered limestone, 4–pyroxene-garnet-phlogopite-tremolite altered limestone, 5–pyroxene altered

limestone plus magnetite-pyrite altered zones, 6–magnetite plus minor pyroxene altered limestone, 7–

magnetite altered zone, 8–feldspar±biotite altered shale, 9–magnetite altered zone, 10–

calcite±magnetite±humite altered limestone, 11–pyroxene altered limestone, 12–pyroxene and quartz

altered sandstone including feldspar±biotite altered shale, 13–calcite-magnetite and humite-forsterite

altered limestone.

skarn

shl

shl

shl

lst lst lst sst

lst unk unk

lst lst lst lst

sst

unk

1 2 3 4

5 6 7 8 9

10 11 12 13

hornfels

hornfels

sandstone

sandstone

hornfels fault

limestone

fault

skarn

skarn breccia

limestone


Figure 4-1 Stratigraphic patterns identified in drilling

Stratigraphic interpretation of drilling from each drill station is illustrated using holes that dip

approximately 60º toward southwest, though significantly steeper holes (KL30-5, KL42-3) were included as

there were no other satisfactory holes from these stations. The position of the upper Waripi sandstone

member relative to the drill collars illustrates continuity of stratigraphy and the basic orientation of strike.

An arrow marks the start position of out-of-sequence stratigraphy. See Appendix I for the position of each

of these drill holes. There is a large amount of vertical exaggeration as the holes are spaced approximately

100m apart (see Chapter 1), meaning the holes represent 1,200m of strike length.


Large-scale geometry

The Waripi and Ekmai Limestone units, as well as upper sections of the Ekmai Sandstone,

dominate the stratigraphy of the main mineralised zone northeast of the Idenberg Fault Zone

(Figure 4-2, Figure 4-3). Much of the rock mass intersected on the southwest side of the Idenberg

fault zone is undifferentiated limestone at higher levels, though it is likely to be the Kais

Limestone, due to the local recognition of the Sirga Sandstone (Figure 4-2). However, the

recognition of the upper Waripi sandstone member in much of the deeper drilling allows some

reconstruction of the geometry of stratigraphy in the footwall of the Idenberg Fault Zone. Bedding

strikes consistently at ~290° in this part of the system. Unit boundaries dip north but are concave

upwards directly adjacent to the Grasberg Igneous Complex, which is intersected at the centre and

northwest end of the deposit and has a near vertical contact with host rocks (Figure 4-3).

Although data on the southwest side of the Idenberg Fault Zone are scarce, the strike of the upper

Waripi shale/sandstone marker is observed to be similar to that on the northeast side. The Ekmai

Limestone is thicker in the southeast than in the northwest, while the Waripi Limestone is thicker

in the northwest than in the southeast (Figure 4-3). Thickening is coincident with inflections in the

strike of the Ekmai Limestone. The distribution of marker horizons in cross section, especially the

upper Waripi shale/sandstone marker, illustrate that total vertical separation across the Idenberg

Fault Zone is ~600m, north side up relative to south (Figure 4-3). The Idenberg Fault Zone has an

offset geometry when viewed in cross section, plan and long section (Figure 4-3). The 290°

striking segments are 50-100m thick, while vertical, 300° striking segments are only 5-10m thick.

(Overleaf): Figure 4-2 is a series of sections of each cross section studied during this research program.

They are included to demonstrate the continuity of stratigraphy as well as the variation in exposure scale

for each section. The stratigraphic patterns are derived from projection of drill traces onto flat page and so

may not be strictly accurate dur to non-planar drill traces. The relative position of the shale horizon marker

in the upper Waripi Limestone gives an impression on the scale of displacement across the Idenberg Fault

Zone. Stratigraphic unit codes are; Tngk = Kais Limestone, Tngs = Sirga Sandstone, Tngf = Faumai

Limestone, Tngw = Waripi Limestone, Kkel = Ekmai Limestone, Kkes = Ekmai Sandstone. Pink shaded

areas depict areas where stratigraphic assignment is not possible, while dotted pattern depicts fault breccia

zones.


Figure 4-2 Serial sections of Kucing Liar lithology (refer Chapter 1 for section locations)


Figure 4-2 Serial sections of Kucing Liar lithology (cont.)


Figure 4-3 Interpretative cross sections of Kucing Liar stratigraphy from wireframes

Cross section KL22

SW NE

Waripi limestone

Ekmai limestone

Ekmai sandstone

Idenberg Fault Zone

Offset fault zone

No characteristic texture or layer to

identify strata

Cross section KL32

SW NE

Waripi limestone

Ekmai limestone

Ekmai sandstone

Idenberg Fault Zone

Waripi sandstone m

ember

Waripi sandstone m

ember

Grasberg Igneous Complex

Buckling of

stratigraphy adjacent to GIC

Different sandstone

layers –must be

separated by fault

Bulging and offset fault zone

Kais limestone

Waripi limestone

Faumai limestone

Sirga sandstone

Cross section KL42

SW NE

Waripi limestone

Ekmai limestone

Ekmai sandstone

Idenberg Fault Zone


Waripi sandstone member


Kais limestone

Buckling of stratigraphy

adjacent to GIC

Bulging and offset

fault zone

Waripi limestone

Faumai limestone

Approximate location only

Three representative cross sections

selected from the centre and either end

of the deposit (spaced roughly 500m

apart, see Figure 4-4). Unlike the

previous Figure 4-2, the stratigraphic

contacts in this figure are not

projections but are sections taken from

individual wireframes that were

interpreted for each unit contact from

its real position in each drill trace

using 3D mine environment software.

The drill traces are projected onto

section planes as grey lines. No

vertical exaggeration. (a) A cross

section through station KL22 shows

relatively simple stratigraphic

succession adjacent to a discrete fault

offset (dotted line). Due to carbonate

alteration and little exposure the exact

position of the stratigraphy to the left

(southwest) of the Idenberg Fault Zone

(IFZ) could not be determined. (b) A

section through KL32 in the centre of

deposit shows a much greater

exposure of the system. In this section

the offset in the IFZ is much more

pronounced. More drilling past the

IFZ has allowed identification of the

upper Waripi shale-sandstone marker

horizon and subsequent overall

movement on the fault zone (heavy

black arrow) (c) A section through

KL42 again shows the IFZ offset and

displacement (heavy black arrow), as

well as the position of the Grasberg

Igneous Complex (GIC). Note the

apparent buckling of stratigraphy

adjacent to the GIC.

(a)

(b)

(c)


Figure 4-4 Interpretative plan sections of Kucing Liar stratigraphy from wireframes

LS2

LS3

LS4

Plan section PS13000mRL


Faumai limestone

Waripi limestone

Idenberg Fault Zone


Kais limestone

Low-angle truncation of

stratigraphy


LS2

LS3

LS4


Waripi limestone

Ekmai limestone

Idenberg Fault Zone

Grasberg

Igneous Complex

Kais or Faumai

limestoneConsistent stepping of

unit contactsVariable

thickness and strike of

fault zone

Low-angle truncation of stratigraphy

LS2

LS3

LS4


Ekmai sandstone

Ekmai limestone

Idenberg Fault Zone


Ekm

ai li

mes

tone

Waripi sandstone

member

Different strike of same stratigraphic

unit

Offset of stratigraphy

orthogonal to

primary displacement

Bulging and pronounced

offset on fault zone

Waripi limestone

Faumai

limestone

Three representative plan sections

spaced 250m apart, see Figure 4-3

for locations. The stratigraphic

contacts in this figure were created

from individual wireframes that

were interpreted from each cross

section. The locations of drill

stations for each cross section are

projected onto the sections for

orientation purposes. The outline of

the Grasberg Igneous Complex

(GIC) was supplied by PT Freeport

Indonesia geologist Peter Manning.

(a) A plan section through 3000mRL

(above sea level) shows the angular

relationship between sedimentary

units and the Idenberg Fault Zone

(IFZ).The IFZ in this section is

relatively simple. The sandstone-

shale marker horizon in the Upper

Waripi Limestone is shown

intersecting the IFZ in the far west.

(b) A section through 2750mRL

(above sea level) shows the

truncation of the Ekmai Limestone

against the IFZ. Also apparent in the

east are offsets in the Ekmai

Limestone that may be an expression

of minor faulting associated with the

IFZ. The IFZ shows variable

thickness and orientation in this

section. (c) A plan at 2500mRL

(a.s.l.) shows a complicated

orientation of the Ekmai Limestone

and a very narrow but offset IFZ.

This section also indicates an offset

of the stratigraphy in the footwall of

the IFZ which is normal to the main

fault zone.

(a)

(b)

(c)


Figure 4-5 Interpretative longitudinal sections of Kucing Liar stratigraphy from wireframes

Long section LS2

NW SE

Idenberg Fault Zone

Upper Warip

i sandstone

Location of contact between two units

is unknown


Waripi limestone

?Faumai limestone

?Kais limestone

Raft of Ekmai limestone in fault zone

Long section LS3

NW SE

Waripi limestone

Ekmai limestone


Ekmai sandstone

Waripi

limestone


Idenberg Fault Zone

Inflection in bedding plunge

Bulge in unit thickness

Bulging offset fault zone

Consistent thickness of stratigraphic unit

along strike

?Faumai limestone

Unknown position or attitude of contact

Long section LS4

NW SE

Waripi limestone

Ekmai limestone


Ekmai sandstone

Subparallel low-angle offsets in

stratigraphy

Significant thinning of stratigraphic unit

Consistent thickness of stratigraphic unit

Three representative

long sections of the

stratigraphic wireframes

created from 3D drilling

data. Intersections of

stratigraphic wireframes

with the section plane

are indicated by

continuous lines. The

sections are vertical and

orientated

perpendicular to the

primary drilling azimuth

(see Figure 4-3 and 4-

4). (a) Is generally in

the footwall of the

Idenberg Fault Zone

(IFZ) which can be seen

in the far left of the

section. (b) A section

through the middle of

the deposit shows

thickening of the Ekmai

Limestone as well as a

bulging offset in the IFZ

which is not equivalent

to that observed in cross

sections in Figure 4-3.

The total displacement

across the IFZ is

indicated by a thick

double-headed arrow.

(c) A section close to the

GIC (see Figure 4-3)

shows relatively simple

stratigraphy with minor

offsets and thinning.

(a)

(b)

(c)


4.1.2 Hydrothermal mineral distribution

This section covers the local-scale controls on fluid infiltration as well as the deposit-scale

controls. The structural controls of fluid flow within Kucing Liar are analysed via meso- (hand

sample), macro- (single drill hole) and mega-scale (drill fan) patterns of hydrothermal mineral

development. Local controls are determined by down hole plots of mineral abundance and

lithological data while deposit-scale controls are identified by comparing alteration distribution

models with lithological models presented in the previous section.

Patterns of hydrothermal alteration

Fluid flow was not uniform through Kucing Liar wall rocks, as fluids were structurally controlled

at various scales. The patterns of mineral distribution are illustrated by down hole logs which

display the abundances of each hydrothermal mineral (Figure 4-6). The three examples shown

illustrate respectively, the Idenberg fault zone in relatively unaltered hosts (KL32-01), a complex

fault system and complicated stratigraphy (KL32-04), and a simple fault system accompanied by

simple stratigraphy (KL32-05). KL32-01, KL32-04 and KL32-05 were all drilled toward 219º at

0, 45 and 60º respectively. In KL32-01, sedimentary rocks on the north side of the fault are

generally unaltered except for low abundance calcite ± magnetite alteration of the upper Waripi

sandstone member that elsewhere commonly contains quartz alteration. KL32-04 was drilled at -

45° and intersected two fault zones delineated by zones where the lithology is generally

unrecognisable. KL32-05 was drilled at a steeper angle and intersected deeper sections of the

Idenberg Fault Zone. Discrete intersections dominated by single minerals extend in length from 5-

100m along a drill hole and are commonly 10-20m. Contacts between such zones dominated by

different minerals are commonly sharp. Some exceptions include gradual and sympathetic

abundance changes between K-feldspar and quartz alteration in the Ekmai Limestone (KL32-04,

350-400m). However, contacts of the K-feldspar-quartz with magnetite-sulphide and

clinopyroxene are commonly sharp.


Figure 4-6 Lithological patterns and mineral abundances in representative drill holes

KL32-01 Idenberg Fault in relatively unaltered hosts

This figure is intended to demonstrate three patterns of lithology and alteration encountered in drilling

conducted on the same cross section (KL32). See Figure 1-12 for the precise angular relationships between

each drillhole. Ornamentation is based on identified lithology while the unit codes are interpreted based on

sequence of lithology. See Appendix V for mineral abbreviations and details of logging process.

Stratigraphic unit codes are; Tngw = Waripi Limestone, Tngl = undifferentiated New Guinea Limestone

Group limestone, Kkel = Ekmai Limestone, Kkes = Ekmai Sandstone. Dashed lines mark the upper and

lower boundaries of unrecognised lithologies that are interpreted to represent fault zones. An asterisk is

used to identify the location of the upper Waripi sandstone member, which is used to establish the total

vertical offset.


Figure 4-6 (cont.)

KL32-05 Simple faulted stratigraphic sequence


Figure 4-6 (cont.)

KL32-04 Complex fault and offset stratigraphy


Large-scale mineral distributions

Clinopyroxene ± garnet skarn (see Chapter 3) is developed as thick (~20-50m-scale) lenses in the

lower Waripi Limestone and Ekmai Limestone, parallel to the folded stratigraphy and are

apparently truncated up dip by the Idenberg Fault Zone (Figure 4-7). Bodies with moderate to

high preserved abundances of skarn-related minerals form a series of stacked lenses that occupy

the lower Waripi and Ekmai Limestones, paralleling the bedding (Figure 4-7). Semi-concordant

skarn rocks in the southeast of the deposit are concentrated in the lower Waripi Limestone and

subordinate positions in the Ekmai Limestone (Figure 4-7). Skarn alteration does not persist to the

northwest past the apparent truncation and thinning of the Ekmai Limestone (Figure 4-7). Early

skarn is zoned with garnet occupying the primary channelways surrounded by clinopyroxene (cf.

small-scale features illustrated in Section 3.2.1). Similarly, a large skarn body dominated by

garnet occurs within the Idenberg Fault Zone. Skarn alteration maintains a constant thickness of

50m for 500m along strike, and maintains a constant position 50m above the base of the Waripi

Limestone. The low density of data in these regions did not allow confident constructions of

volumetric models for skarn mineral development in the footwall sequence to the southwest of the

Idenberg Fault Zone.

Volumes of preserved moderate abundances of K-feldspar ± biotite are tightly restricted to the

Ekmai Limestone (Figure 4-8). In cross section, K-feldspar ± biotite rocks are concentrated

wholly within the Ekmai Limestone. Biotite alteration is most extensive where the Ekmai

Limestone is thickest but is also concentrated in deeper portions of the Ekmai Sandstone and

associated with the Idenberg Fault Zone (Figure 4-8). By contrast, moderately magnetite-rich

rocks are prominent as a single concentration 20m thick along the base of the Waripi Limestone,

extending along most of the identified strike extent (Figure 4-9). Significantly, the magnetite

rocks extend into the Grasberg Igneous Complex where they appear to be portioned at the

Grasberg Igneous Complex boundary, the first alteration to appear as such (Figure 4-9). At the

deepest levels in the down-faulted stratigraphic package, magnetite is concentrated in limestone,

assumed to be Faumai Limestone, above the upper Waripi sandstone member (Figure 4-9).


Retrograde skarn minerals tremolite-actinolite and serpentine have a similar distribution to

magnetite, though details of any stratigraphical or structural control are not visible.

Quartz alteration is concentrated into stratigraphic layers abutting the Idenberg Fault Zone (Figure

4-10). Well-defined bodies of quartz-dominant material 10-50m thick were identified in drill core

and are found to extend up to 500m along strike (Figure 4-10). Quartz alteration is less well

developed in the east than in the west. Quartz alteration also occurs as a discrete package in the

upper Waripi sandstone member. Moderate to high abundances of sulphides are concentrated

within the Idenberg Fault Zone (i.e. broadly coincident with quartz alteration) (Figure 4-11).

Additional smaller concentrations of sulphides are present along major stratigraphic contacts,

particularly the Ekmai Limestone contacts. Sulphide concentrations are continuous for hundreds

of metres along strike (Figure 4-11). Sulphide development is not continuous from the Idenberg

Fault Zone to the Grasberg Igneous Complex. The independent development of chalcopyrite and

covellite-bearing mineralisation is reconfirmed in models of their spatial distribution (Figure 4-

11). Distributions of covellite-bearing mineralisation are distinctly concentrated about the

Idenberg Fault Zone as well as in the adjacent Ekmai Limestone. In contrast, chalcopyrite-bearing

mineralisation is concentrated along the Ekmai Limestone and is continuous into the Grasberg

Igneous Complex.


Figure 4-7 Distribution of calcite, clinopyroxene, garnet, humite and phlogopite

Sections of skarn alteration

differentiated by the various

minerals identified in

Chapter 3. The Grasberg

contact was provided by

Freeport geologists.

(a) A cross section through

station KL32 shows the

stratiform nature due to

lithological layer control of

clinopyroxene and garnet

accumulations as well as

zoning pattern of garnet

inside clinopyroxene. (b) A

plan section through

2,750m demonstrates that

the skarn development does

not envelop the GIC. The

asymmetric zoning of skarn

across the IFZ relative to

elevation is also clear. (c) A

long section perpendicular

to cross sections

demonstrates once more the

stratiform nature of skarn

as well as the zoning

pattern grading from

proximal garnet to

clinopyroxene to distal

calcite ± magnetite. See (a)

& (b) for location.

(a)

(b)

(c)


Figure 4-8 Distribution of dominant K-feldspar + biotite alteration

Sections of the distributions

of the various potassic

alteration group minerals

identified in Chapter 3. The

Grasberg contact was

provided by Freeport

geologists.



stratiform nature due to

lithological layer control of

K-feldspar and biotite

accumulations. Deeper

sections of Kucing Liar are

biotite rich about the IFZ.

(b) A plan section through

2,750m demonstrates the

lithological control as well

as a suggestion of zoning

from inboard biotite to

more distal K-feldspar. (c)

A long section

perpendicular to cross

sections demonstrates once

more the lithological

control highlighted in

Chapter 3 as well as the

zoning pattern grading

from proximal biotite to

more distal K-feldspar. See

(a) & (b) for location.

(a)

(b)

(c)


Figure 4-9 Distribution of extensive magnetite and tremolite-actinolite alteration


of magnetite and retrograde

skarn alteration identified

in Chapter 3 demonstrate

the strong lithological (or

stratigraphic contact)

control on magnetite and

retrograde skarn and also

that retrograde skarn and

some magnetite is not

stratiform. The Grasberg

contact was provided by

Freeport geologists.


station KL32 shows that a

large amount of magnetite

is juxtaposed with the GIC.

The section demonstrates a

hydrothermal connection

between the GIC and

Kucing Liar during

magnetite alteration (b) A


2,750m demonstrates the

same connectivity with the

GIC. (c) A long section

perpendicular to cross-

sections demonstrates once

more the strong

stratigraphical control. See


(a)

(b)

(c)


Figure 4-10 Distribution of rocks dominated by quartz, muscovite and anhydrite alteration


of quartz, anhydrite and

talc-muscovite alteration

identified in Chapter 3

demonstrate weak

lithological control on

quartz alteration but the

disparate nature of this

apparently temporally

related assemblage.


station KL32 shows well the

lithological control on

quartz, where the

stratigraphy is adjacent t

the IFZ. The upper Waripi

sandstone member has

strongly partitioned some

quartz alteration. Anhydrite

distributions are very

similar to that of tremolite-

actinolite but show no clear

structural control. (b) A


2,750m suggests a parallel

structure at the margin of

GIC has concentrated

quartz alteration. (c) A long

section perpendicular to

cross sections demonstrates

once more the strong

stratigraphical control. See


(a)

(b)

(c)


Figure 4-11 Distribution of ore sulphides, pyrite, chalcopyrite and covellite


of pyrite, chalcopyrite and

covellite demonstrate the

strong influence of the IFZ

and more subtle lithological

control on ore minerals as

well as a zoning pattern

about the IFZ of proximal

covellite and distal

chalcopyrite.



intensity of pyrite and

covellite development in the

IFZ offset as well as the

influence of the Ekmai

Limestone. (b) A plan

section through 2,750m

shows the most intense

pyrite alteration is opposite

the GIC. Note that the

contact zone of Kucing Liar

and the GIC where

magnetite and retrograde

skarn are concentrated is

occupied by high

chalcopyrite

concentrations. (c) A long

section perpendicular to

cross sections further

demonstrates the zoning

pattern of ore minerals. See


(a)

(b)

(c)


4.2 LARGE-SCALE CONTROLS ON FLUID INFILTRATION

The data in this chapter will show that the Kucing Liar hydrothermal system was related to a

major structural offset in the Idenberg Fault Zone, which is adjacent to a significant lithological

contrast.

4.2.1 Structural geometry of Kucing Liar alteration

The combination of specific rock types and marker horizons (Chapter 2) has enabled construction

of a lithological model for the mineralised zone. Models of these data indicate that Kucing Liar

lies within the north dipping limb of a syncline, although no fold closures are evident in the study

area. Adjacent to the Grasberg Igneous Complex the bedding is folded against the intrusion

contact, suggesting forceful intrusion. The host stratigraphy has been truncated at a very shallow

angle to strike by a steeply dipping fault zone. The fault zone is named the Idenberg Fault Zone

and contains several steeply northeast dipping narrow structures that are connected by wide zones

of brecciation. The zone of displacement follows both the narrow structures and wide zones to

produce a series of offsets within the fault zone. The displaced portion of Kucing Liar on the

southwest of the Idenberg Fault Zone is difficult to analyse due to very low data densities. The

same rock types are encountered in the footwall of the Idenberg Fault Zone, though skarn is more

prevalent than other alteration types.

The mineral distribution data indicate the Idenberg Fault Zone focussed the entire system while a

series of complex offsets in the fault zone provided local controls, specifically on garnet and

sulphide distributions. Specific alteration assemblages are concentrated along the lower Waripi

and Ekmai Limestone contacts, as well as within the Idenberg Fault Zone, especially within

offsets of the fault. Within the mineralised zone hydrothermal alteration occupies the upper

sandstone member of the Waripi Limestone, the lower Waripi Limestone, the Ekmai Limestone

and also extends downwards into the Ekmai Sandstone. Skarn alteration tends to be stratiform and

is concentrated in the Ekmai Limestone and lower half of the Waripi Limestone. Humite-forsterite


± serpentine and clinopyroxene ± tremolite-actinolite are restricted to the dolomitic Waripi

Limestone (see Chapter 2) within the main mineralised zone and appear to stratiform and

interlayered, perhaps reflecting the original distribution of dolomite and calcite in the limestone

unit. Garnet and magnetite are localised within the Waripi Limestone along its lower contact with

the Ekmai Limestone and to a lesser extent along the base of the Ekmai Limestone. Small

concentrations of garnet are also localised along the upper skarn contact within the Waripi

Limestone. K-feldspar ± biotite, along with related quartz veins, is generally restricted to the

Ekmai Limestone and Ekmai Sandstone though biotite also formed independently within narrow

portions of the Idenberg Fault Zone below the elevation of the main mineralised zone. Quartz and

sulphide alteration have very similar distributions that appear to parallel the steeply dipping

structures within the Idenberg Fault Zone and are concentrated about a large-scale offset in the

fault zone. Quartz and sulphide are structurally distinct from other alteration assemblage, as they

do not form large stratiform bodies. The change in alteration distribution from skarn to potassic to

silica-pyrite indicates a change in structural controls that will be analysed in the next section. The

relationship between chalcopyrite and covellite mineralisation in Kucing Liar and the Grasberg

porphyry system has not been comprehensively tested, though the two systems have similar ore

assemblages, there are some grounds for believing the two are distinct systems, and will be

further discussed in Chapter 9.

The data indicate mineralisation that is zoned with respect to fluid flow. Mertig et al., (1994),

Hefton et al., (1995), and Rubin and Kyle (1998) have described vertical zonation of alteration

and mineralisation in the magnesian skarn deposits of the EESS, formally referred to as GBT-

IOZ-DOZ (see Chapter 1). The focus of fluid flow at Kucing Liar was the Idenberg Fault Zone,

and in particular offset within it, and fluids probably flowed upwards and along stratigraphic

contacts to that feature. Fluids may then have migrated within the Idenberg Fault Zone to higher

elevations. In a model where covellite formation is at least partly contemporaneous with, though

spatially distinct from, chalcopyrite, the data suggest that chalcopyrite ± pyrite was accompanied

by and locally overprinted by covellite ± pyrite, which is restricted to the high flow areas. Both of


these forms of copper mineralisation were replaced in the core of the Idenberg Fault Zone by a

package of pyrite ± chalcopyrite ± covellite. Pyrite, chalcopyrite and covellite core are

overprinted by galena and sphalerite.

4.2.2 Driving forces of fluid flow

Fluid infiltration through rocks may be via primary or secondary porosity. Primary porosity is a

function of the grain size, degree of cementation and distribution of the wall rocks, while

secondary porosity is that which is created during deformation or alteration in the absence of

deformation. The very fine-grained texture of rock samples, particularly pyroxene and feldspar,

indicate derivation from rapid deposition at numerous nucleation sites, which can result from high

fluid fluxes that are conducive to supersaturation (Einaudi et al., 1981). Additionally, pervasive

fluid flow such as is observed to have occurred during skarn and potassic (K-feldspar ± biotite)

alteration is inferred to occur along microcracks and grain-boundary porosity (Oliver, 1996).

Pervasive fluid flow produces uniform replacement of wall rocks, referred to as penetrative

alteration (Chapter 3). Widespread penetrative alteration is indicative of low fluid pressures and

will typically be associated with relatively high fluid fluxes as compared to channelled flow

(Oliver, 1996). Channelled fluid flow occurs along fractures in wall rocks but is accompanied by

substantial infiltration into the local wall rocks, typically resulting in a mineralogical selvedge

(Oliver, 1996). The progressively declining scales of penetrative alteration accompanied by

increased fracture selvedge and infill indicate that fluid flow became more and more channelled

accompanied by increasing fluid pressures. There are also indications that the amount of

channelled fluid flow increased with time, evidenced by the increase in infill relative to alteration

and the decrease in penetrative alteration in later stages of the paragenesis (Chapter 3).

Within a fault zone, fluid migration occurs from zones of high interstitial pressure and high strain

(contraction zone) to zones of low interstitial pressure (dilation zone) (Guha et al., 1983). Flow

localization within faults and shear zones occurs in areas of highest fracture aperture and fracture

density, such as damage zones associated with fault jogs, bends and splays (Cox et al., 2001).


Offsets are thus favourable sites for fluid flow due to complex geometry created by the large

amount of wall rock partings and intersections of variably oriented fractures. Fluid flow in a fault

network is governed by creation of permeability through movement. Where high fluid pressures

produce low effective confining pressures, grain scale crack growth significantly increases the

permeability of the active shear zone relative to their host rocks (Cox et al., 2001). Thus,

secondary permeability is created by high pore fluid pressure regimes, which favour fracture

growth (Cox et al., 2001). Mineral-filled fractures in hydrothermal systems indicate tensile

effective stress states, and thus, fluid pressures greater than σ3 (lithostatic load) (Cox et al., 2001).

Sustained hydrothermal flow must be accompanied by repetitive and continued wall rock

fracturing given that mineral sealing is rapid compared to the lifetimes of hydrothermal systems

(Cox et al., 2001). Consequently, sustained fluid flow occurs only in active structures where

permeability is repeatedly renewed. Fault motion is accommodated by earthquake-related

rupturing (Sibson, 2001) and is accompanied by significant fluid redistribution that occurs

throughout the aftershock phase following large earthquakes (Cox et al., 2001). Secondary

porosity related to lithological layering may also be produced during folding as deformation of

heterogeneous rocks creates dilatancy due to competency contrast, as well as large variations in

pore fluid pressure (Pf), leading to brecciation along these contacts (Oliver et al., 2001).

Thus deformation can explain brecciation along the base of the Waripi Limestone. In similar

fashion to Kucing Liar, the Big Gossan deposit is concentrated in breccia bodies within the lower

Waripi Limestone near the contact with the Ekmai Limestone, which was altered to pyroxene-

feldspar and biotite-feldspar hornfels and also contains local garnet-pyroxene skarn (Meinert et

al., 1997). The preference for the Ekmai Limestone as a host for quartz vein arrays may also be

derived from ground preparation due to contact metamorphism of the shaly limestone, as brittle

calc-hornfels are easily fractured during deformation (Einaudi et al., 1981).

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